Steam line plugs are installed in the steam line nozzles of boiling water reactors for inspection and maintenance of the steam line system of the reactor. Generally, the plugs are of two types. One type of plug is only used to prevent the steam line from being filled with water. A positive head of water on the reactor side of the plug keeps the plug inserted in the steam line nozzle while the line is drained. The plug may contain a vent valve operated remotely to allow the steam lines to refill after maintenance. This type of plug, however, cannot withstand the back pressure which results from the local leak rate testing of the main steam valve closest to the plug. A force of approximately 50 psi will force the plug from the steam line nozzle.
Another type of plug has been developed to withstand the back pressure from the local leak rate testing of the main steam valve closest to the plug. In order to absorb the load resulting from the back pressure and remain in place in the steam line, however, this plug must incorporate an elaborate expansion device. The expansion device may include hydraulically expanded rings to lock the steam line plug against back pressure. It is, thus, a problem in the prior art to provide a steam line plug which is of simple design and does not require elaborate expansion devices, and yet is able to withstand the local leak rate test pressure and remain in place in the steam line.